PRETORIA, South Africa – It was a good day in court for Oscar Pistorius.

After 19 days of presenting evidence, Oscar Pistorius' defense finally got to the crux of its case with its – allegedly – last witness.

Sports Medicine Professor Wayne Derman, seemingly the defense's star expert witness in the Paralympian's murder trial, says people with disabilities live under greater threat of attack, and have a different fight-or-flight response to able-bodied adults.

It is crucial testimony in support of the athlete, who maintains that when he shot and killed his girlfriend Reeva Steenkamp in the early hours of Valentine's morning last year, he believed her to be an intruder and was trying to protect them both.

In a blow for the prosecution, the findings of psychologist Jonathan Scholtz – one of the mental health experts who conducted Pistorius' 30-day psychiatric assessment – also supports Derman's testimony.

"When Mr Pistorius's appraisal of the situation is that he might be physically threatened, a fear response follows that might seem extraordinary when viewed from the perspective of a normal-bodied person, but normal in the context of a disabled person with his history," the 50-page report reads.

Pistorius' "putative self defense" strategy relies on his ability to demonstrate that he genuinely feared for his life when he fired his 9mm pistol four times through a locked toilet door.

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Oscar Pistorius hugs a supporter in the Pretoria High Court. (Getty Images)

If he persuades Judge Thokozile Masipa that his fearfulness as a result of his disability made shooting a "reasonable" response, Pistorius could be found guilty of "culpable homicide" – a charge comparable to manslaughter in the U.S., and which carries no mandatory prison sentence – instead of murder.

Derman told the court of Pistorius' "very exaggerated startle response," describing it as "excessive." He said when confronted with fireworks at opening and closing ceremonies for athletic competitions, the Paralympian would cover his head and ears, "cowering away."

"I have observed a significantly higher startle response in persons with disability, reacting to perceived threat and imminent fear of harm," Derman said on the stand.

Pistorius ground his teeth silently in the stand. A man who never wanted to be defined by his disability, his defense strategy now rests on his vulnerability.

The prosecution has argued throughout the trial that the double-amputee's declared "instinct" to confront the intruder instead of seeking safety elsewhere, was irrational and improbable. Derman disagrees.

"The fight-or-flight response, which is a primitive reflex, is often exaggerated and prominent in people with disabilities. A reflex is something you don't have control over," he said, comparing Pistorius' desire to approach his perceived intruder to a reported incident of a mother defending her children against an attacking polar bear.

Scholtz's report also suggests that the Paralympian's direct and indirect exposure to crime throughout his life contributes to his assessment of situations.

The defense hammered the point home, leading evidence from Derman on two studies which conclude that in the United States, disabled people are twice as likely to be victims of crime, and a WHO report that suggests adults with varying levels of disability are at higher risk of violent attacks. Derman said Pistorius' disability increased his risk of attack by some 31 percent.

Pointing at a diagram of the human brain, Derman explained that anxious people have less control over their subconscious "fight-or-flight" response to dangerous situations. The sports physician described Pistorius as a "very anxious individual" with a hand tremor, adding that he was medicated for a sleep disorder.

In a study of stress in disabled athletes, the professor quoted Pistorius' score – significantly higher than the group average.

Derman – a member of the International Paralympic Committee's Medical Commission – says crimes against the disabled increased in the aftermath of the London 2012 Paralympics, of which Pistorius was a star.

The sports physician has been a long-time supporter of the athlete. In 2007, Derman supported the Blade Runner's bid to compete against able-bodied athletes, describing him as "an icon of humanity in rising above adversity, and on that level I'm behind him all the way."

Derman also served as a Medical Officer for the South African team at the Sydney and Athens Olympics and Beijing and London Paralympics, saying he gained "intimate knowledge" of Pistorius while they resided together in Athletes' Villages.

He gave the court graphic descriptions of the medical problems the double-amputee encountered, including testifying about a "very distressed Mr. Pistorius" showing him his bleeding stumps on a video Skype call from London while he was trying to qualify for the Games.

Scholtz's report again echoes the point, finding that Pistorius has "a history of feeling insecure and vulnerable, especially when he is without his prostheses."

The independent report is an unexpected boon for the defense.

It confirms that Pistorius has no history of "abnormal aggression or explosive violence," nor does he "display the personality characteristics of narcissism and or psychopathy that are mostly associated with men in abusive relationships."

Scholtz writes that "there is evidence to indicate that Mr. Pistorius was genuine with his feelings towards Miss Steenkamp and that they had a normal loving relationship."

"Although the relationship was still young, there were no signs of abusive coercion like those often found in these kinds of relationships," the report reads.

In his testimony, Derman told the court of a telephone conversation with Pistorius, 12 days before the shooting. "He said he was lying next to the most wonderful girl he had ever met," he told the court. "He wanted me to meet her."

The report describes the murder-accused as "severely traumatized," now suffering from a post-traumatic stress disorder and a major depressive disorder. It prescribes that Pistorius "should continue to receive clinical care by a psychiatrist and a clinical psychologist for his current condition," or it may worsen and "increase the risk for suicide."

But amid the numerous findings that may help the defense, the prosecution will no doubt make much of the definitive conclusion that Pistorius did not have generalized anxiety disorder when he shot Steenkamp, as alleged by a defense forensic psychiatrist.

The state's case now rests on its ability to prove that Pistorius consciously and purposefully fired his 9mm Taurus.

In a highly controversial turn of events, Judge Masipa granted a court order barring the publication of any further details of the hefty psychiatric report, despite it purportedly being a public document and having already been released to the media.

Despite the seeming advantages to the defense's case for the report to be in the public domain, Pistorius' advocate Kenny Oldwadge told the court he had been instructed to request the restriction, citing family privacy concerns.

Masipa, herself a former crime journalist, handed down the order, which also makes any individual who retweets the previously reported details guilty of contempt of court.

Derman returns to the stand Thursday, to continue with his evidence in chief.